The 3rd Generation Partnership Project (3GPP) has standardized Enhanced uplink for WCDMA. Among the features introduced are fast scheduling and fast Hybrid Automatic Repeat Request (HARQ) with soft combining, both located in the Node B. An illustration of the overall architecture is illustrated in FIG. 1-A, with the new features introduced by the enhanced uplink in FIG. 1-B.
Hybrid ARQ with soft combining allows a radio base station (120), i.e. the Node B in a WCDMA-system, to rapidly request retransmissions of erroneously received data entities, leading to significantly reduced delays compared to earlier releases of the WCDMA specification according to which the Radio Network Controller (RNC) 110 is responsible for all retransmissions within the radio access network. Soft combining with hybrid ARQ can also be used to enhance the capacity of the system by deliberately targeting multiple transmission attempts for each data entity, and by using the soft combining mechanism in the receiver to accumulate the received energy until the data is successfully decoded. This can be viewed as implicit link adaptation and was not provided for in earlier releases of the WCDMA specification due to the lack of a soft combining mechanism. Typically, a small number of transmission attempts (i.e., a low Block Error Rate (BLER) for the initial transmission), reduces transmission delays at the cost of a decreased system capacity. Similarly, by targeting a larger number of transmission attempts (i.e., a high BLER for the initial transmission attempt), the system capacity is increased at the cost of increased delays. The choice of hybrid ARQ operating points (in terms of the targeted number of transmission attempts) thus depends on the system load and the delay requirements for a particular service.
For each transmission attempt in the uplink, a positive or negative acknowledgement is sent in the downlink. If there are several users in difficult radio conditions (e.g. at the edge of the cell), a simultaneous transmission of acknowledgements to these users can consume a significant amount of the total Node B power.
Uplink transmissions on the Enhanced Dedicated Channel (E-DCH) can use any of the transport formats defined for the E-DCH. With each transport format, there is an associated power value, typically specified relative to the Dedicated Physical Control Channel (DPCCH), and a payload size. Fast scheduling denotes the possibility for the Node-B to control when user equipment (e.g., a Mobile Terminal) is transmitting and at what data rate. This influences the selection of the transport format that the user equipment should use. Data rate and transmission power are closely related, and scheduling can thus also be seen as a mechanism to vary the transmission power used by the user equipment for enhanced uplink traffic on the Enhanced Dedicated Physical Data Channel (E-DPDCH). As the power availability in the user equipment at the time of transmission is not known to the Node-B, the final selection of data rate has to be performed by the user equipment itself. The Node-B only sets an upper limit on the transmission power the user equipment may use on the E-DPDCH. If a user equipment identifies that it does not have sufficient power available to fully utilize the upper limit given by the Node-B, it will limit the data rate. At extreme cases, the user equipment may determine that it does not have sufficient power to transmit even a single Protocol Data Unit (PDU).
To ensure that important information, such as radio resource control signalling, can always be transmitted, a minimum set is defined. The minimum set can always be transmitted, even if Node B has not scheduled the user equipment. Also, if the power available would not be sufficient to guarantee a required error probability, the minimum set can be transmitted and the retransmission protocol (HARQ or Radio Link Control (RLC)) would ensure that the packet is eventually delivered. Typically, the minimum set would consist of transport format(s) carrying only a single PDU.
In 3GPP2, similar enhancements have recently been introduced for cdma2000. The concept of minimum set for Release 99 (R99) is defined in 3GPP TS25.331, incorporated herein by reference.